In modern day computer systems, a central processing unit, or CPU, possesses instructions and data, most of which, due to the main storage limitations within the CPU, are stored in one or more peripheral storage devices external to the CPU. A CPU may be connected to a data channel which, in turn, is connected to the peripheral storage device by way of a storage control unit. Alternately, the CPU may be directly connected to the storage control device by ways of controlled circuitry internal to the CPU itself.
One type of peripheral storage device in common use is the magnetic disk storage drive. In the manufacture of disks for such devices, the yield is such that a certain number of disks manufactured will contain surface defects which, in the past, render the disk unfit for data recording. In the more recent past, such defective disks were rendered usable by flagging data tracks which contain such defects and recording data normally assigned to these defective tracks on alternative tracks. Each defect, therefore, disabled an entire data track which was then flapped as defective, even though the defect may have been contained in only a relatively small area of the track. This solution was wasteful of space, and furthermore, there were only a few alternate tracks available for defective track assignments. Therefore, the number of disk defects affecting unique tracks that could be tolerated was equal to the number of alternative tracks available for this solution.
Another approach to avoid the defects in the recording media in a disk storage system, was to allocate a space record length on each data track for the purpose of recording a record that would be affected by a defect in its normal recording area. This approach called for the records being of fixed length and for each track to have a spare record for possible use. The defective record on each track was identified by a flag bit at the beginning of that record which in turn caused the record to be automatically transferred to the spare record for that track. In order to practice this method of defect avoidance, the user must give up a percentage of recording area equal to one record length. Therefore, if there were to be five records on each data track, then 20% of the usable area on the disk would be dedicated to the defect avoidance scheme.
Another type of peripheral storage device is the commonly used magnetic tape storage drive. Classicly, defect avoidance has taken the approach of having a spare track on the tape which carries information as to the availability of the magnetic surface of an area across the tape as defined by each data bit in the avoidance track. Therefore, in using the method, one must give up one recording track in order to provide the defect avoidance scheme. This defect marker inhibits the writing or reading of any information in all data tracks across the magnetic tape whenever the defect mark is sensed during a read or write operation. Therefore, data tracks that did not have the defect associated with it are affected the same as if the defect was in fact present. This approach by use of a special defect track, provides for the avoidance of multiple defects per track along the length of the magnetic tape.
Another approach within the magnetic tape art for defect avoidance is that when a defect is encountered within a record, the magnetic tape be backspaced to the beginning of the record that is affected and then a certain length of tape is erased to form a gap and thereafter the record is re-recorded. If the defect still affects the newly recorded record, then the tape is again backspaced to the beginning of the now re-recorded record and again the same length of area is erased to provide an extension of the original erased area. The record is then again re-written and this process continues until the record is so placed that the defect does not affect the record. The area of magnetic recording surface that is utilized in this defect avoidance scheme is a variable according to where the defect is located with regard to the record being recorded. It can be understood that if the defect is at the end of a very long record, the erased area would in necessity have to be almost as long as the original record length in order to avoid this defect. This method calls for a trial and error approach for avoiding the defect and can be a time consuming process.
Accordingly, it is the primary object of my invention to provide control apparatus for controlling the read and write operations of a storage system to cooperate with the initialized magnetic medium such that the greatest efficiency of magnetic media surface usage may be obtained while making the system transparent to defects during a read/write operation.
Another object of my invention is to provide a method which dynamically allocates the reserved special gap such that defects on the recording media will be placed in such a relationship with the special gap that the defect will not affect usable information recorded on the recording media.
Another object of my invention is the method of avoiding defects in a record during a read operation and for predicting the distance to the defect for the next record to be read or written during the read processing of that record.
It is yet another object of my invention to provide apparatus and method for making the most efficient utilization of the system time by allowing the location of the defect to be predicted and therefore avoiding the trial and error scheme of previous systems.